In this study, we evaluated the activities of new types of azaphenothiazines in the following immunological assays: the proliferative response of human peripheral blood mononuclear cells induced by phytohemagglutin A or anti-CD3 antibodies; lipopolysaccharide-induced cytokine production by human PBMC; the secondary, humoral immune response in mice to sheep erythrocytes (in vitro); and delayed-type hypersensitivity in mice to ovalbumin (in vivo). In some tests, chlorpromazine served as a reference drug. The compounds exhibited differential inhibitory activities in the proliferation tests, with 10H-2,7-diazaphenothiazine (compound 1) and 6-(3-dimethylaminopropyl)diquinothiazine (compound 8) being most suppressive. Compound 1 was selected for further studies, and was found to be strongly suppressive in the humoral immune response even at low concentrations (1 microg/ml). Compound 1 also inhibited the delayed-type hypersensitivity lipopolysaccharide-induced production of tumor necrosis factor and interleukin-6 in cultures of human blood cells. As there were only two subjects in this study, the effects of these compounds on human blood cells need to be confirmed. In this paper, we also discuss the structure-activity relationships of selected compounds.